The present invention relates to the field of polymer films and, more particularly to a uniaxially heat shrinkable biaxially oriented polypropylene film.
As noted in U.S. Pat. No. 4,194,039, polyolefins can be used to prepare shrink films for wrapping purposes. Other suitable synthetic resins include various ionomers, polyvinyl chlorides, polyesters, polystyrenes and polyvinylidene chlorides.
A shrink film's distinguishing characteristic is its ability upon exposure to some level of heat to shrink or, if restrained, to create shrink tension within the film. This ability is activated by the packager when the wrapped product is passed through a hot air or hot water shrink tunnel. The resulting shrinkage of the film results in an aesthetically pleasing transparent or opaque wrapping which conforms to the contour of the product while providing the usual functions required of packaging materials such as protection of the product from loss of components, pilferage, or damage due to handling and shipment. Typical items wrapped in polyolefin shrink films are toys, games, sporting goods, stationery, greeting cards, hardware and household products, office supplies and forms, foods, phonograph records, and industrial parts.
The manufacture of shrink films requires relatively sophisticated equipment including extrusion lines with "racking" capability, irradiation units when cross-linking is desired, tenter frames, mechanical centerfolders, and slitters. "Racking" or "tenter framing" are conventional orientation processes which cause the film to be stretched in the cross or transverse direction and in the longitudinal or machine direction. The films are usually heated to their orientation temperature range which varies with different polymers but is usually above room temperature and below the polymer's melting temperature. After being stretched, the film is rapidly cooled to quench it thus freezing the molecules of film in their oriented state. Upon heating, the orientation stresses are relaxed and the film will begin to shrink back to its original, unoriented dimension.
Certain applications, e.g., labelling, covering, or packaging of materials such as boxes, plates, vessels, bottles, tubes, cylindrical material, e.g., pipes, and rods, etc. are especially susceptible to covering with heat shrinkable films. However, in certain situations it is desirable to effect shrinkage along a single axis without substantial shrinkage in the cross-direction. For example, in the process of labelling bottles by shrinking a tube of heat shrinkable material, if the film shrinks along its length, the label may not be placed in the right position but rather placed above the desired position upon shrinkage. Moreover, printing and other conversion processes of such label surfaces require heat stability in substantially one direction to meet machinability requirements. Uniaxially shrinkable materials can also be used in preparing tightly wrapped containers by lap heat sealing or lap hot melt adhesive sealing uniaxially shrinkable film resulting in shrink down of the wrapping.
In order to obtain uniaxially shrinkable materials it is possible to employ uniaxially oriented materials, i.e., materials which are oriented in only one direction. However, uniaxially oriented film can lack the requisite strength and toughness necessary for use in such applications. Inasmuch as biaxially oriented films exhibit desirable strength and tear resistance in both directions of orientation, it would be desirable to obtain a uniaxially heat shrinkable film which is biaxially oriented and thus substantially stable in the cross-direction.
For more detailed disclosures of heat shrinkable films, reference may be had to aforesaid U.S. Pat. No. 4,194,039, as well as U.S. Pat. Nos. 3,808,304; 4,188,350; 4,377,616; 4,390,385; 4,448,792; 4,582,752; and 4,963,418, all of which are incorporated herein by reference.
U.S. Pat. No. 5,292,561 (corresponding to EPA 0498249) discloses a process for producing polyolefin shrink films having high unidirectional shrinkage (at least 10% longitudinal shrinkage and less than 2% transverse shrinkage at 100.degree. C.) under conditions comprising an MD reorientation and mechanical MD/TD draw ratio between 1.01 and 7.5. The base layer of the films contain propylene polymer and optionally, hydrogenated hydrocarbon resin.
EPA 0204843 discloses a low temperature shrinkable film comprising linear low-density polyethylene resin having film shrink properties of 30% or more MD and 5% or less TD at 90.degree. C., which is prepared by drawing the film at a high draw ratio (3 to 6) in the machine direction.
EPA 0321964 describes a process for extruding a shrink film from a linear low density copolymer of ethylene and at least one alpha-olefin having 3 to 6 carbon atoms to provide a material which exhibits shrinkage at 135.degree. C. of at least 30% MD and at least 10% TD.
EPA 0477742 discloses a transparent polypropylene shrink film which exhibits shrinkage at 100.degree. C. of at least 10% MD and less than 2% TD. The polypropylene comprises a 15% or less, preferably 2 to 6% n-heptane soluble component.
EPA 0299750 discloses a mono- or biaxially stretched film having a heat shrinkage of 20% or more in one of the longitudinal and transverse directions and 60% or more in the other direction. The film comprises principally a linear polyethylene and optionally, a branched low-density polyethylene.
EPA 0595270 discloses a heat sealable laminate having high unidirectional shrinkage. produced from biaxially oriented polymeric film such as biaxially oriented polypropylene or blends of polypropylene and copolymers of propylene with minor amounts of ethylene or an alpha-olefin. Uniaxial shrinkability is achieved by balancing MD reorientation process variables such as temperature, draw ratio, line speed, and oriented polymer film properties. Heat sealability is imparted by the presence of a heat seal layer.
Generally, the desired high level of MD shrinkage at label application temperatures (above about 130.degree. C.) is accompanied by proportionally high MD shrinkage at storage temperatures. High MD shrinkage at storage temperatures results in roll tightening and ensuing film distortions and blocking. Another consequence of high MD shrinkage at storage temperatures in the case of film stored after printing is loss of accuracy in the MD label repeat distance.
It is the object of the present invention to provide a biaxially oriented multilayer film comprising a polypropylene core which is uniaxially heat shrinkable with long term stability.